Hi, i have a kolsh recipe that has a OG of 1.052 and FG of 1.012. I calculate 4.2% abw & 5.25% abv using a couple of other methods and the beertools shows 4.07% abw & 5.19% abv. Does anybody have any idea why there are differences?

I just set up a recipe in BTP to test this and got 4.1% abw and 5,24% abv, by entering the values into the analysis tab. You will get very slightly different values, as you got, if you use the efficiency and attenuation fields to get to the OG and FG, due to rounding errors.

For example, 1.0515 and 1.0124 would appear as 1.052 and 1.012, but would give 4.02% abw and 5.12% abv, while 1.0524 and 1.0115 would also appear as 1.052 and 1.012, but give 4.21% and 5.35%, respectively.